Resonant tunneling diodes (RTDs) are used as memory devices due to the bistable behavior that results from a hysteresis intrinsic to the DC current-voltage characteristics of the RTDs. This bistability is predicted to exist in nanoscale devices such as single electron transistors and single molecule transistors. Tunneling current through degenerate states of a single quantum dot or molecule leads to a switching effect only in the case of an attractive electron-electron interaction which is mediated by the electron-phonon interaction. It has been proposed, considering only a single energy level, that the hysteresis of I-V characteristics can be observed in a single molecule junction with an effective attractive Coulomb interaction on the basis of the Hartree approximation and the polaron effect. Hysteretic tunneling current in a polaron model has also been observed beyond the Hartree approximation.
Theoretical studies have predicted the existence of hysteresis in a quantum dot or molecular junction, although conclusive experimental support for the predictions has not been achieved. The tunneling current through a carbon nanotube quantum dot exhibits a periodic oscillatory behavior with respect to an applied gate voltage, which arises from an eightfold degenerate state. A periodic oscillatory differential conductance also arises as a result of a tunneling current through a single spherical PbSe quantum dot having a sixfold degenerate state. However, neither of these situations exhibits a bistable tunneling current, indicating that electron-phonon interactions in nanotube quantum dots and PbS quantum dots are not sufficient to yield the strong effective electron-electron interactions necessary for the existence of a bistability. In general, bistable current in existing memory systems arises from a phase transition of a bulk material, which phase transition vanishes on the nanoscale.
Semiconductor quantum dot arrays can be chemically fabricated to form a superlattice. The size of the quantum dots and the lattice constant of the superlattice are controllable via nanoscale manipulation, enabling charges in the quantum dot array to be tuned in either the Coulomb blockade regime or the semiconducting regime. Consequently, quantum dot arrays are promising candidates for the investigation of strongly correlated systems as well as for use as integrated electronic devices.